newbie, laptop spec

evening all.
im a new meber form the UK.
was wondering what an ideal spec is for a laptop to be used as a carpc.
i need a middle between performance and efficiency.
whats sort of processor, ram should i be looking for?

Think of it this way:
You have certain tasks you want the PC to perform. So look at hardware configurations that are capable of providing the performance you desire for the tasks you intend to perform.

You last para of questions...
No - you can never have overkill. It's like saying you have more space than you can ever fill.
You can never use too much power or RAM nor have too much heat etc.
Unless it costs too much, or your air con can't keep up (or have a 2 door whose 2WD only cools half as much as 4WD) or you battery flattens too quick or your alternator is too small.
RAM size is usually not that relevant anyhow unless running on laptop batteries. Despite claims to the contrary, RAM does not consume hundred of Watts - they usually mean milli-Watts.

A higher spec machine may consume less power despite higher speed & more RAM... but I'm interpreting "spec" as in performance (and maybe price).